Several device designs have been made for filtering a feed liquid into a filtrate container. These are typically used to clarify and sterilize biological solutions, such as fetal calf serum, tissue culture media and the like. They may be an open device, meaning that their supply is open to the atmosphere or they may be a closed type of device in which the system is closed to the atmosphere. In an open device, the user must transfer the feed liquid from a storage vessel to the filter device, thus adding an added manual step to the process. Open filtration devices include the Stericup™ device available from Millipore Corporation of Billerica, Mass. which can handle a maximum unfiltered volume of 1 liter based on the size of the feeding funnel. The advantage of a closed filtration device is that it can process larger volumes continuously, as determined by the volume of the feed and filtrate storage vessels. Also, there is no manual transfer from the storage vessel to the closed filter assembly.
Closed devices generally include three components; a container for the unfiltered material, a container for the filtrate and a vacuum driven filter assembly between the two containers. In most of these systems, the first container is a flask or the package in which the unfiltered liquid is shipped (typically a bag or plastic bottle). The second (filtrate) container is a glass bottle or flask. The filter assembly is a separate essentially closed system having an inlet that is in fluid communication to the first container, typically by tubing, air-tightly sealed to the first container, a filter downstream of the inlet and an outlet downstream of the filter. The outlet is in fluid communication with the second container either directly or by a hose or tubing and is air-tightly sealed to the second container. A vacuum port is arranged downstream of the filter so as to draw the fluid from the first container, through the filter and into the second container.
The filter assembly typically has a gasket area designed to fit on top of the second container so as to establish a seal for the vacuum that drives the filtration process.
If, during operation, air should enter the closed filter assembly or if the feed is accidentally stopped, such as by the feed tube coming out of the feed liquid, filtration is stopped. Reestablishing the liquid flow can be difficult to achieve. Liquid flow will not be re-established until the wetted filter, closed to air bypass, can again be in contact with the unfiltered liquid stream.
Such devices have, in one instance, incorporated two filters, one hydrophilic to allow for the passage of liquid and one hydrophobic to allow for the passage of air. An alternative, as shown by U.S. Pat. Nos. 4,036,698 and 5,141,639, is to use a single membrane containing both hydrophilic areas and hydrophobic areas to allow for the passage of both air and liquid through different areas of the same membrane.
Both methods facilitate restarting the filtration by vacuum by allowing both air and liquid to pass simultaneously through the filter device.
These devices have several disadvantages.
First, the use of two membranes (one hydrophilic and the other hydrophobic) requires a unique design and sealing of two separate filters in place.
Likewise, the use of a hybrid membrane (part hydrophilic, part hydrophobic) is expensive and requires a determination of the proper balance of the respective philic/phobic areas. Too much phobic area reduces the filtration rate adversely. Likewise, too little phobic area makes restarting as difficult as if no phobic region existed.
Additionally, the phobic membrane or area is in contact with the fluid being filtered and will over time wet out or become clogged or fouled with contaminants or product such as proteins. This results in an irreversible air locking upon an air incursion as the phobic area no longer functions.
Both versions are difficult to integrity test with standard bubble point procedures due to the presence of the philic/phobic membranes at the same time.
Another problem with current closed filtration devices is that these devices are sensitive to small intrusions of air because the upper housing chamber is too small to hold any air away from the membrane, and therefore relies on the hybrid membrane to provide the sole means for evacuating trapped air. When the phobic vent is fouled as can occur with larger volume filtration, continuous filtration is over and the device must be replaced.
What is needed is a simpler closed filtration device for vacuum filtration. What is needed is a single closed filtration device that can process larger volumes of liquid from a feeding vessel without stopping under normal conditions due to air entrapment and can be purged of air in the event of stoppage to keep from having to discard and replace the device. Also desirable is a device that self seals to the filtrate container such that it can run unattended without stoppage due to a fouled phobic vent and small incursion of air.